1. Field of the Invention
The present invention concerns a method for the welding of two metallic parts by a laser beam, when there is a clearance between the parts, needed for their assembly.
2. Description of the Prior Art
Welding by laser beam is a commonly used method that gives excellent results if the laser is powerful enough, and if the parts are abutting and clean. This method is used, for example, to weld the lid laid on a package for the encapsulation of semiconductor components. The components may be sensitive to heat. This fact rules out the use of heat to cause the melting of a solder preform. However, they have to be enclosed in imperviously sealed chambers to ensure that they are protected. In this case, laser beam welding is a convenient method if the lid, placed on the package, abuts it: a laser discharge parallel to the plane of the lid and in the joint plane provides for perfect and impervious welding.
However, in certain configurations, there may be a clearance between the two parts to be welded. This is the case, for example, if the lid of a package is embedded in the grooved sides of this package. The precision with which the parts are machined results in a small clearance between these parts. This small clearance is necessary to enable the handling of the parts before their final soldering. Experience shows that, in this case, if the laser beam is discharged in the joint plane, there are always welding defects, and leakages, especially in the corners.
The method for the laser beam welding of two parts with a clearance between them consists in performing two successive laser discharges. A first discharge, done outside the joint plane, makes one of the two parts melt locally. The collapsing of the molten material puts the two parts into contact with each other, and removes the clearance at the bead of the collapsed material. A second discharge, which itself is also done outside the joint plane but is closer to the joint plane than the first discharge, does the welding between the two parts which no longer have any clearance between them.